IV 



AECHINEPHRIC DUCT 



239 



of the archinephric duct which is relatively primitive. In a number 

 of Vertebrates there appear to be distinct traces of the formation of 

 the front end of the archinephric duct by fusion of the outer ends 

 of tubule rudiments in a manner essentially the same as that which 

 holds for Hypogeophis. As will have been gathered from the pre- 

 ceding pages this is the case with such different groups of Vertebrates 

 as Elasmobranchs, Crossopterygians, Lung-fishes, Eeptiles and Birds. 



end 



^-ecl. 



y — ^y, 



Ok 





-f.c. 





cf 



I X^; 



- 



* '^ W^j 



a. 



/0.n.c 



Fig. 181. — Diagram illustrating a possible mode of evolution of the archinephric duet. 



A, the coelomic compartments are bulging towards the nephridial tubes ; B, the compartments 

 have come to open into the nephridial tubes and the flame-cells have disappeared ; C, D, the external 

 openings of the nephridia are becoming shifted backwards so as to give rise to the archinephric duct ; 

 E, the archinephric duct is completely formed and communicates with the enteron through one of 

 the segments retaining, or reverting to, its primitive enterocoelic connexion. a.n.d, archinephric 

 duct; c.f, coelomic funnel; cod, coelomic cavity; cl.o, cloacal opening of archinephric duct; ect, 

 ectoderm ; end, endoderm ; /.c, flame-cell ; n, nephridial tube. 



If we are justified in looking upon this mode of formation of the 

 duct in ontogeny as relatively primitive, it obviously affords strong 

 support to the second of the two above-mentioned hypotheses as to 

 the evolutionary origin of the archinephric duct : the bending back 

 of the tubule rudiments would then be interpretable as a develop- 

 mental reminiscence of the backward shifting of their external open- 

 ings which took place during phylogeny. 



